Methodology

Every number on this site should be checkable. This page explains where the figures come from, what we assume, and what we refuse to guess at.

Dimensions

All quantities are calculated from actual surfaced (S4S) lumber dimensions, not nominal labels. A 2x6 is 1.5" × 5.5"; a 2x8 is 1.5" × 7.25"; a 2x10 is 9.25"; a 2x12 is 11.25". A 5/4x6 deck board is 1" thick and 5.5" wide.

Coverage

Board coverage includes the installation gap. A 5.5" board with a 1/4" gap covers 5.75" of surface. Row counts are rounded up, because you cannot install a partial row.

Waste

Waste is a function of the layout, not a flat percentage:

• Straight — 10%. Square cuts, minimal offcut loss.
• Diagonal (45°) — 15%. Both ends of every board are cut at an angle.
• Picture frame — 18%. Adds mitred perimeter boards on top of the field.

Loose fasteners carry a separate 5% spares allowance.

Counts

Counts are geometric, never rules of thumb:

• Joists: floor(span ÷ spacing) + 1. The +1 closes the far end.
• Fastening points: board rows × joists — the number of places a board actually crosses a joist.
• Footing volume: πr²h, converted to 80 lb bags at 0.60 cu ft yield each.
• Balusters: spaced so a 4" sphere cannot pass, per IRC R312.1.3. We design to a 3.75" gap so site tolerance cannot push it over the limit.
• Stair risers: chosen near a 7.5" target, then increased until every riser is within the 7.75" maximum of IRC R311.7.5.1.

What we withhold

Some numbers are structural-safety decisions rather than arithmetic, and we will not publish them until we can stand behind them.

• Joist and beam spans. Withheld until our tables are transcribed and double-checked line by line against the published AWC DCA6 tables. Until then the calculator says so rather than printing a plausible-looking figure.
• Frost depth. Footing depth defaults are placeholders. Required depth ranges from nothing in the far south to over 100" in the far north, and only your local building department can tell you which applies.
• Ledger fastener schedules. Ledger attachment is the most common point of deck failure. The fastener pattern and flashing detail must come from DCA6 or your local code official.

Sources

• International Residential Code (IRC) — R311.7.5 stairs, R312.1 guards.
• American Wood Council DCA6, Prescriptive Residential Wood Deck Construction Guide.
• Manufacturer installation instructions for composite decking joist spacing.

Corrections

If you find a formula you believe is wrong, tell us and show your working — we will check it and publish a correction. Email hello@cutandcount.com.

This site provides planning estimates. It is not engineering advice and does not replace a permit, an inspection, or a licensed professional.